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Like all the guys said before - dots -. I investigated this problem myself from the web sites usability point of view. It was just as you described it, European use commas and the rest of the world uses dots.
I used the "dot" myself in many ecommerce sites. But, it you still need - or just wanna use commas - make it clear obvious for the users what does the "comma" represent between numbers.

Sometimes, you might need to use the both of them, for ex: 15,250,600.50 where the commas are used to seperate the thousand's amount and the dots are used for decimal amount. Same can be written as 15.250.600,50 - which I never recommend to - but it still give the same info.

You can use whatever you prefer, but make sure to declare in written text to all your users what does those -dots or commans- means!

Why call them periods when you can call them "full stops?" (I just read an article on alistapart.com about em and en dashes and all sorts of wierd typography stuff).

When I first saw this post it really threw me off. I mean, I never really even considered commas to be an option. Hey, I just thought of what they are really called:

decimal point noun (ca. 1864)
: a period, centered dot, or in some countries a comma at the left of a proper decimal fraction (as .678) or between the parts of a mixed number (as 3.678) expressed by a whole number and a decimal fraction
(c)1999 Prentice-Hall, Inc. All rights reserved

Originally posted by JMulder EDIT: five40, you actually need to press the VOTE button on the top of the post to vote

Yes, I know. I just skipped the voting part and wanted to say that we use commas too.
Actually the reason why I didn't vote is because I seem to get error message about invalid session every time I try to do that.

Surprise, surprise, I use comma's . The problem lies not in number like 7,2 / 7.2; it lies in the bigger numbers with a few decimals as well.
The number with either a decimal or a number bigger then 999 (which would need either a comma or a period) without any decimals, are pretty easy to understand for all people. It sometimes takes a person a while to get what is meant, but usually the message gets through.

The problems lies with what Sash already explained, the use of both in one number. This could cause enormous confusion.

As for the usuability point of everything, I think periods would be the way to go.

Yes, I know. I just skipped the voting part and wanted to say that we use commas too.
Actually the reason why I didn't vote is because I seem to get error message about invalid session every time I try to do that.

grrr i hate those error messages... EVERYTIME i vot i have to click the link like 3-4 times before it accepts that i do want to vote.